Marine heat waves (MHWs) are a major threat to gorgonian coral forests, causing widespread bleaching and mortality events. While the shallow waters where these corals typically reside are highly vulnerable to MHWs, the deeper waters of the seabed may provide a refuge from these extreme temperature events.

Mechanisms for resilience:

1. Depth-related temperature differences: The temperature difference between the shallow and deep waters can be significant. During MHWs, the surface waters can experience extreme temperature increases, while the deeper waters remain relatively cooler and more stable. This temperature gradient allows gorgonian corals in deeper environments to avoid the lethal conditions experienced in shallow waters.

2. Upwelling events: Upwelling events, which bring cold, nutrient-rich water from the deep ocean to the surface, can mitigate the effects of MHWs. During these events, the cooler water from the seabed rises and mixes with the warmer surface waters, helping to lower the overall temperature. This cooling effect can provide relief for gorgonian corals that are exposed to high temperatures in shallow waters.

3. Hydrodynamic conditions: The seabed often experiences different hydrodynamic conditions compared to shallow waters. Deeper environments can have reduced wave energy and stronger currents, which can promote the mixing of water masses and enhance nutrient availability. These favorable conditions can support coral growth and resilience during MHWs.

Examples:

Several studies have demonstrated the protective effects of the seabed for gorgonian coral forests during MHWs:

- In the Mediterranean Sea, gorgonian corals located at depths below 30 meters were less affected by a severe MHW in 2003 compared to those in shallower waters.

- Off the coast of California, gorgonian corals in deeper habitats were found to have higher survival rates during the MHW of 2014-2016, highlighting the potential of the seabed as a refuge.

Implications for conservation:

The identification of the seabed as a potential refuge for gorgonian corals during MHWs has important implications for conservation efforts. By prioritizing the protection of deep-sea habitats where gorgonians can find refuge, we can enhance their resilience and increase their chances of survival during future extreme temperature events.

However, it is crucial to note that the effectiveness of the seabed as a refuge may vary depending on the specific characteristics of the MHW, the depth range of the gorgonian corals, and the local environmental conditions. Further research is needed to better understand the dynamics and implications of seabed refuges for gorgonian conservation in different regions and ecosystems.